Type S or type VS: how to pick the right safety washer, and fit it properly

The difference between safety washer type S and type VS is thickness and the bolt grade it suits. Type S covers bolt sizes M1.6 to M36 for normal duty with property classes up to 8.8. Type VS is thicker for higher pre-tensioning loads, suits high strength bolts of grade 10.9 without restriction, fits M5 to M30, and is the type to use on elongated holes and against hardened surfaces. Both share the same inside and outside diameters and the same serrations.

Most of the questions we take in Ann Arbor are some version of the same one: which washer, and does it go in the right way round. Both are worth answering properly, because the wrong choice or a careless fit undoes the locking effect you paid for.

Type S: the standard choice

The type S washer is the original SCHNORR safety washer for normal requirements. It is available for bolt sizes M1.6 to M36 in property classes up to 8.8, which covers the majority of general machine building and industrial assembly.

Its outside diameter is matched to the head diameter of pan head and socket head cap screws, so it sits under the head without overhanging the bearing face. If your joint is grade 8.8 or below, on a normal clearance hole, in a material of ordinary hardness, type S is the part.

Type VS: for high strength bolts and awkward joints

Type VS is the heavy duty version. It has greater thickness to take higher pre-tensioning loads, which is what makes it the washer for grade 10.9 bolts and high strength connections. It is available in sizes M5 to M30, and the outside and inside diameters and the serrations are identical to type S.

Three situations point to VS even when the bolt grade alone would not:

• Slots and elongated holes. A bolt on a slot with no securing device does not necessarily loosen when tightened fully, but it damages the bolt head considerably and needs a high initial torque. With a type VS washer the required initial torque is around 25 per cent lower, the securing effect is good, and damage to the head is minor. If you are looking for a washer for slotted holes, this is it.
• Hardened contact surfaces. As the hardness of the base surface increases, the serration bites less, and securing performance follows that bite. SCHNORR washers are 39 to 45 HRC (380 to 450 HV30), so against a hard surface we recommend the heavy duty type VS.
• Higher preload targets. Where the design leans on the top of the bolt’s capability, the thicker section handles the load without flattening out.

How to install a safety washer, and the three ways people get it wrong

Fitting is simple, but there is only one correct arrangement. To install a safety washer, place a single washer directly between the bolt head or nut and the clamped component, serrations against both faces, cone facing so the serrated crown bears on the surfaces it needs to bite. Then tighten to the full specified preload. Three habits to avoid:

• Do not stack washers. Two washers on top of each other cannot bite into anything except each other.
• Do not add a plain washer. A flat washer under the safety washer gives the serrations a loose interface to work against and removes the mechanical locking half of the effect.
• Do not fit it upside down. The cone has an orientation, and reversing it changes how the load is carried.

Two useful fitting notes: safety washers work on clearance holes when there is a washer on both sides of the joint, and they are designed to be used in counterbores.

Then choose the material and finish

Once the type is settled, the material follows the environment. C60S carbon steel to DIN EN 10132-4 is the standard, used exclusively for serrated safety washers and load washers. X5 CrNiMo 17-12-2 (1.4401) austenitic stainless steel to DIN EN 10151 handles corrosive service. CuSn8 bronze to DIN EN 1654 is the option where magnetism is a problem, with the caveat that its strength values and spring forces are considerably lower than standard material.

Finishes include blackened, mechanical zinc, zinc flake, nickel plated and electroless. On securing effect, a black surface performs best, but a zinc coated washer secures a joint perfectly well provided the bolt is tightened with the appropriate initial force. Zinc plated parts also need no increase in initial torque.

A quick specification checklist

• Bolt size and property class, which decides S or VS.
• Hole type: clearance, counterbore or slot.
• Hardness of the bearing surface.
• Environment: corrosion, temperature, magnetism.
• Finish, and whether the assembly torque allows for it.

Bring those five answers to a conversation and the part number falls out in a couple of minutes. The Product Finder on schnorr.com covers standard sizes, and CAD drawings are available for the model.

• SCHNORR safety washer type S covers bolt sizes M1.6 to M36 in property classes up to 8.8.
• Type VS is thicker for higher pre-tensioning loads, suits grade 10.9 high strength bolts without restriction, and fits M5 to M30.
• Type S and type VS share the same inside and outside diameters and the same serrations.
• On slots, type VS needs about 25 per cent lower initial torque than an unsecured bolt and causes only minor bolt head damage.
• Washer hardness is 39 to 45 HRC (380 to 450 HV30). Type VS is recommended against hardened surfaces.
• Do not stack safety washers, combine them with a plain washer, or fit them upside down.
• Materials include C60S carbon steel, X5 CrNiMo 17-12-2 stainless and CuSn8 bronze. A black finish gives the best securing effect.